Method and apparatus for timing recovery in ADSL transceivers under a TCM-ISDN crosstalk environment

ABSTRACT

A system used to synchronize the clock frequency of a receiver with that of a transmitter, where both the receiver and transmitter communicate using the ADSL Annex C standard. The transmitter continuously transmits pilot tones to the receiver. The receiver determines the phase error between sequential pilot tone symbols to determine a phase error. The receiver uses the phase error to adjust the receiver clock frequency. However, if the most recently received symbol is subject to near end crosstalk or is subject to far end crosstalk and is a boundary symbol, the receiver ignores the phase error and does not adjust the receiver clock frequency.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.09/515,415, filed Feb. 29, 2000, now U.S. Pat. No. 6,724,849, which isincorporated by reference in its entirety.

BACKGROUND

1. Field of the Invention

The present invention relates generally to digital subscriber linetransceivers for communicating using telephone subscriber loops, andmore particularly, to techniques for performing clock frequencysynchronization (timing recovery) for asymmetric digital subscriber linetransceivers under TCM-ISDN crosstalk.

2. Background of the Invention

FIG. 1 depicts in a block diagram a relationship between a singletransmitter 102 (central office (CO)) and single receiver 104 (customerpremises equipment (CPE)) that use digital subscriber line (DSL)communications over copper telephone wires 106. The wider bandwidthneeded for DSL transmission generates crosstalk interference amongcopper wire pairs bundled in the same cable binder. The level ofcrosstalk varies for different cable structures and materials. Somecountries such as Japan and Korea use telephone cables with apaper-based “pulp” insulator rather than the plastic insulated cables(PIC) used in the United States. These pulp cables have high level ofcrosstalk between different services over copper wires bundled in thesame cable binder. ISDN service has been deployed widely over copperwires. Crosstalk caused by ISDN service is one of the majorinterferences to other newly deployed DSL services since portions of thetransmission band for ISDN service overlap with portions of thetransmission band for DSL services.

In countries such as Japan, where the noisy pulp cables are installed, aspecial TCM-ISDN system is deployed. This system is described in theInternational Telecommunication Union Telecommunication StandardizationSector (ITU-T) specification G.961, Appendix III. The G.961 Appendix IIIsystem reduces crosstalk interference by switch synchronizing ISDN cardsat the central office using Time Compression Multiplexing (TCM). TCMprovides for ISDN signal transmission and reception during differenttime periods to reduce near-end crosstalk between ISDN services.

ITU-T ADSL standards G.992.1 and G.992.2 Annex C (hereafter “ADSL AnnexC”) describe the operation of DSL modems under TCM-ISDN interference.Signal transmissions from DSL modems are switch synchronized to a 400 HzTCM Timing Reference (TTR) generated at the central office. The TTRsignal is the master clock signal for determining when the centraloffice modem (the “CO 102 modem”) and the customer premises equipment(the “CPE 104 modem”) should transmit and receive ISDN and DSL signals.

Within the same cable binder, TCM generates a time varying noiseenvironment. During the first half period of the TTR signal, the COmodem is dominated by near end crosstalk (NEXT) interference, androughly speaking, during the second half period the CO modem isdominated by far end crosstalk (FEXT) interference. The reverse is truefor the CPE 104 modem. FIG. 2 is a diagram illustrating the relationshipbetween TTR, ISDN, and G.992.2 timing.

The TCM-ISDN crosstalk environment can be different depending on thelength of the subscriber loop. On long subscriber loops, because thereceived signal is heavily attenuated, the NEXT interference is largecompared to the received signal. The channel capacity in the NEXT periodcan be greatly reduced, sometimes be zero. On the other hand, in theFEXT period, the channel typically has good signal-to-noise ratio (SNR)because the FEXT interference is much weaker than NEXT, and smallrelative to the received signal.

FIG. 3 illustrates the relationship between the TTR signal, the ISDNNEXT/FEXT interference, and ADSL Annex C transmit frames. A “SlidingWindow” operation specified in G.992.1 and G.992.2 Annex C defines theprocedure for transmitting symbols under ISDN interference synchronizedto the TTR signal. The FEXT_(R) symbols are symbols completely insidethe FEXT_(R) period. The NEXT_(R) symbols are symbols inside any portionof the NEXT_(R) period. Thus, there are more NEXT_(R) symbols thanFEXT_(R) symbols, as shown in FIG. 3. The CO modem 102 decides if aparticular symbol is a FEXT_(R) symbol or NEXT_(R) symbol according tothe sliding window and transmits the symbol according to bit mapscorresponding to FEXT_(R) and NEXT_(R) symbols. Similarly, the CPE 104modem decides if a particular symbol is a FEXT_(C) symbol or NEXT_(C)symbol and transmits the symbol according to bit maps corresponding toFEXT_(c) and NEXT_(c) symbols. The bit map for NEXT symbols can be allzero. In that case, only one bit map is used in each direction for FEXTsymbols only. Although the exact symbol time is sliding relative to theTTR signal, the pattern is fixed by ADSL Annex C to be periodic with theperiod 345 symbols long, which is hereafter referred to as a“hyperframe.”

Referring to FIG. 4, there is shown the 345 training symbols that makeup a hyperframe, and its relationship to the TTR signal including themapping of NEXT_(R)/FEXT_(R) symbols. The only significant differencebetween the NEXT_(R) and FEXT_(R) symbols is the additive TCM-ISDNinterference. Any symbol that is partially affected by NEXT interferenceis treated as NEXT symbol. The FEXT symbols represent a signal treatedas transmitted entirely during the FEXT period. The remaining trainingsymbols are treated as though they were transmitted during the NEXTperiod. From FIG. 4, it is observed that the TTR signal and the CO modemsymbols are not aligned. However, over a period of 345 symbols, the TTRsignal spans 32 or 34 periods, depending on the cyclic prefix selectedby the CO modem. This least common multiple period is used by ADSL AnnexC to define the hyperframe.

ADSL Annex C specifies a Discrete Multi-tone (DMT) system, whichincludes a plurality of tones having different carrier frequencies, eachof which is modulated with different data. Tone 64 is used to transmit a“pilot tone” which enables synchronization of the clocks of the CO andCPE modems. The pilot tone is transmitted by the CO modem 102 (master)and synchronized to by the clock of the CPE modem 104 (slave). Using aconventional pilot tracking technique, the CPE modem checks the receivedpilot signal continuously to control the CPE modem clock. However, underthe TCM-ISDN interference environment, use of a pilot tone transmittedduring both NEXT and FEXT periods can lead to inaccurate synchronizationof the clocks of the CO and CPE 104 modems because the pilot signalduring NEXT period may be badly corrupted by the TCM-ISDN NEXT.

What is needed is a method and apparatus that improves thesynchronization of the clocks of the CO and CPE under TCM-ISDNcrosstalk.

SUMMARY OF THE INVENTION

One embodiment of the present invention includes an apparatus used toperform timing recovery, namely, synchronize the frequency of thereceiver clock with a reference clock of a remote transmitter. Forexample, in this embodiment, the frequency of the reference clock or arepresentation thereof is transmitted to the receiver. When thisembodiment is used in an environment such as ADSL, near end crosstalkperiodically disrupts the integrity of the reference signal. In thisembodiment, a computer implemented method is provided that includes thefollowing acts. First, receiving a first pilot tone symbol from thetransmitter. Second, receiving a second pilot tone symbol from thetransmitter later. Third, determining a phase error between the firstand second pilot tone symbols. Fourth, selectively setting the phaseerror to zero based on the crosstalk environment of the second pilottone symbol. The TCM-ISDN crosstalk environment is predictable andperiodic. Fifth, adjusting the frequency of the receiver clock based onthe phase error. In this embodiment, the act of setting the phase errorto zero occurs if the crosstalk is near end type. By setting phase errorto zero, clock adjustment is skipped. Alternatively, in this embodiment,the act of setting the phase error to zero occurs if the crosstalk isfar end type, the receiver is in early training mode, and the pilot tonesymbol is a boundary symbol adjacent to a NEXT symbol.

One embodiment of the present invention includes an apparatus thatdetermines a phase error to use to synchronize a receiver clockfrequency with that of a transmitter, where the system includes: atransmitter that transmits at least one pilot tone and a training modeindicator to the receiver; a sample clock that generates a sample clocksignal; a symbol identifier that receives the sample clock signal andthat indicates a symbol index associated with the most recent symbol; aphase error measurer device that measures the phase error between thetwo pilot tone symbols and outputs the phase error; and a phase errordetermination device (“PEDD”) that receives the phase error from thephase error measurer device, the symbol index from the symbol identifierand the training mode indicator from the transmitter, wherein the PEDDdetermines whether to ignore the phase error based on the symbol indexand the training mode indicator. In this embodiment, the PEDD determinesthe crosstalk environment of the most recent symbol by using the symbolindex and ignores the phase error if the crosstalk is near end type. Inone embodiment, the PEDD determines the crosstalk environment of themost recent symbol by using the symbol index, determines whether themost recent symbol is a FEXT boundary symbol and ignores the phase errorif the crosstalk is far end type, the training mode indicator signalsearly training mode, and it is a boundary symbol, namely the FEXT symboladjacent to a NEXT symbol.

Advantageously, the above described embodiments use pilot tone symbolstransmitted in tolerably noisy environment to synchronize the receiverclock with the transmitter clock; this embodiment avoids using signalstransmitted in an intolerably noisy environment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts in a block diagram a relationship between a singletransmitter and single receiver that communicate using the asymmetricdigital subscriber line (ADSL) standard over copper telephone wires.

FIG. 2 is a diagram illustrating the relationship between TTR, ISDN, andADSL Annex C timing signals.

FIG. 3 illustrates the relationship between the TTR signal, the ISDNNEXT/FEXT interference, and ADSL Annex C transmit frames.

FIG. 4 shows 345 training symbols that make up a hyperframe, and itsrelationship to the TTR signal as well as mapping of NEXT/FEXT symbols.

FIG. 5 provides a block diagram of a suitable implementation of a CPE104 in accordance with an embodiment of the present invention.

FIG. 6 depicts in block diagram form a suitable embodiment of theoperation of the clock frequency adjuster of CPE.

FIG. 7 depicts a suitable process 700 executed by the phase errordetermination module.

FIG. 8 specifies which symbols are boundary symbols in early trainingmode.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In one embodiment of the present invention, the CPE 104 is modified todetermine which pilot tone symbols, from the CO 102, to use tosynchronize the clock 105 of the CPE 104 with clock 103 of the CO 102.One advantage of this embodiment is more accurate matching of thefrequency of the clock signals of the CPE 104 and CO 102. However, atrade off is that the clock signals of the CO 102 and the CPE 104 arematched at a slower rate.

FIG. 5 provides a block diagram of a suitable implementation of a CPE104 in CPE 500 that is in accordance with an embodiment of the presentinvention. Each component of the CPE 500 may be implemented in software,hardware, or firmware. Where any part of the CPE 500 is implemented insoftware, a suitable microprocessor, input/output device, and memory areused to execute the software.

The CPE 500 includes a conventional signal extractor 502, conventionalA/D converter 504, a conventional sample clock 506, a conventionalserial-to-parallel converter 508, a conventional linear time domainequalizer (TDE) 510, a conventional fast Fourier transformer (FFT) 512,a clock frequency adjuster 514, which is in accordance with anembodiment of the present invention and described in more detail below,and a conventional phase locked loop (PLL) 516.

Signal extractor 502 performs 2-to-4-wire conversion, which converts thebi-directional two-wire signal 501 transmitted using the copper wireline 106 (FIG. 1) into two pairs of one-directional transmissions. Onepair is for receiving and the other pair is for transmitting.Additionally, the signal extractor 502 can include a splitter thatfilters-out signals from the bi-directional two-wire signal 501 that arenot included in a typical DSL transmission band. For example, the lowfrequency POTS signal is blocked by the splitter so that it will notinterfere with high frequency ADSL signal, as is well known in the art.

The conventional A/D converter 504 receives an analog signal from thesignal extractor 502 and outputs a digital rendition to a conventionalserial-to-parallel converter 508. A conventional sample clock 506 setsthe sample frequency of the A/D converter 504. The conventionalserial-to-parallel converter 508 converts the serial bit stream from theA/D converter 504 into a stream of digital samples and outputs thestream of samples to the time domain equalizer (TDE) 510.

The conventional TDE 510 is, for example, a finite impulse response(FIR) filter that compensates for the channel distortion (e.g.,frequency response and phase distortion over a telephone line) in thetime domain. For a description of a suitable implementation of the timedomain equalizer 510, see U.S. patent application Ser. No. 09/514,976filed Feb. 29, 2000 and entitled “Equalizer Training For ADSLTransceivers Under TCM-ISDN Crosstalk Environment,” which isincorporated herein by reference in its entirety. The time domainequalizer 510 outputs digital samples to a FFT 512.

The conventional FFT 512 converts the digital samples from time domainequalizer 510 to the frequency domain and separates all of thesub-channels. One of the sub-channels is a pilot tone that istransmitted on tone 64 in accordance with G.992.1 and G.992.2. The pilottone is provided to the clock frequency adjuster 514.

The clock frequency adjuster 514, in accordance with one embodiment ofthe present invention, outputs a phase error that controls the frequencyof the sample clock 506. A suitable process of the clock frequencyadjuster 514 is described below and with respect to process 700.

The conventional PLL 516 receives a phase error from the clock frequencyadjuster 514 and adjusts the clock frequency of the sample clock 506based on the phase error.

Clock Frequency Adjuster 514

FIG. 6 depicts in block diagram form a suitable embodiment of theoperation of the clock frequency adjuster 514 of CPE 500. Thisembodiment of the clock frequency adjuster 514 is implemented insoftware that is executed by a microprocessor, or the clock frequencyadjuster 514 can be implemented as hardware or firmware.

The clock frequency adjuster 514 includes a conventional phase errormeasurer module 602, a phase error determination module 604, and aconventional symbol identifier module 606. The phase error measurermodule 602 provides a phase error to the phase error determinationmodule 604. The symbol identifier module 606 receives the sample clocksignal from the sample clock 506 of FIG. 5 and provides a symbol indexto the phase error determination module 604. The phase errordetermination module 604 determines whether to signal the PLL 516 ofFIG. 5 to adjust the frequency of the sample clock 506 to match theclock signal of the clock 103 of the CO 102.

The conventional phase error measurer module 602 receives the pilot tonefrom the FFT 512 (FIG. 5), which is computed once every symbol. Thephase error measurer module 602 measures the phase error between currentand a previously received pilot tone symbols.

In one embodiment, the pilot tone is represented using quadratureamplitude modulation (QAM) as a complex number (i.e., real and imaginarycomponents). Thereby the pilot tone represents a vector when plotted onan X-Y plane. When the receiver clock is not synchronized to the remotetransmitter clock, this pilot vector will rotate from symbol to symbol.The phase error measurer module 602 provides the difference (error) inphase between pilot tone symbols (“phase error”) to the phase errordetermination module 604. The pilot tone symbols may or may not besequential to each other.

The symbol identifier module 606 determines a symbol index, i.e., thesymbol number transmitted in relation to the 345 symbols transmitted ina hyperframe (FIG. 4), associated with the most recently receivedsymbol. In this embodiment, the symbol identifier module 606 firstdivides the frequency of the sample clock signal by 256 for G.992.2 or512 for G.992.1 if the CPE 500 is in early training mode (where cyclicprefix is not added) and otherwise divides the sample clock signalfrequency by 272 for G.992.2 or 544 for G.992.1 for later training mode(“divided sample clock signal”). G.992.1 and G.992.2 specify that afterthe C-SEGUE-1 signal, a cyclic prefix is used. Use of cyclic prefixcorresponds to later training mode.

Next, the symbol identifier module 606 determines the symbol index by,for example, following each period of the divided sample clock signal,incrementing the symbol index by 1 or decreasing the symbol index by 345if the symbol index is greater than 344. The symbol index is initiallyset to zero (0). The symbol identifier module 606 sends the symbol indexto the phase error determination module 604.

The phase error determination module 604 receives the phase error fromthe phase error measurer module 602, the symbol index from the symbolidentifier module 606, and the early training mode indicator from theCPE modem 500 after it detects the end of C-SEGUE1 in a manner wellknown in the art. The phase error determination module 604 determinesthe phase error to apply to the PLL 516 of CPE 500 of FIG. 5. A suitableprocess 700 executed by the phase error determination module 604 isdescribed with respect to FIG. 7.

Process 700

Process 700 decides whether the phase error computed by the phase errormeasurer 602 is reset to 0. If it is not reset, the phase error computedby phase error measurer 602 shall be used by PLL 516 to adjust theclock. In action 710 of process 700, the phase error determinationmodule 604 determines the crosstalk type, i.e., NEXT or FEXT. In thisembodiment, the phase error determination module 604 determines thecrosstalk type by using the symbol index and training mode (i.e., earlyor later). The G. 992.2 Annex C specifies the following formulas todetermine the crosstalk type:

For early training mode (no cyclic prefix),

S=256*N_(dmt) mod 2760   if { (S+255 < a) or (S > a+b) } then FEXTsymbol   else NEXT symbol where   N_(dmt) is the current symbol index,  a=1243, and   b=1461(see C.5.2.2 of G.992.2). For later training mode (with cyclic prefix),

S=272*N_(dmt) mod 2760   if { (S+271 < a) or (S > a+b) } then FEXTsymbol   else NEXT symbol where   N_(dmt) is the current symbol index,  a=1243, and   b=1461(see C.8.1 of G.992.2). For G.992.1, similar formulas can be found inG.992.1 Annex C.

In action 720, the phase error determination module 604 determineswhether the CPE 500 is in early training mode. If the CPE 500 is inearly training mode, action 730 follows, otherwise for late trainingmode, process 700 ends.

In action 730, the phase error determination module 604 determineswhether the symbol index is a “boundary symbol” as defined in G.992.1and G.992.2 Annex C. For example, FIG. C-11 in G.992.2 specifies whichsymbols are boundary symbols in early training mode. FIG. 8 replicatesthe FIG. C-11. As shown in FIG. 8, there are two boundary symbols perrow, where the boundary symbols are FEXT symbols adjacent to NEXTsymbols. The following identifies the boundary symbols by symbol index:0, 3, 11, 14, 22, 25, 33, 36, 43, 46, 54, 57, 65, 68, 76, 79, 87, 90,97, 100, 108, 111, 119, 122, 130, 133, 140, 143, 151, 154, 162, 165,173, 176, 184, 187, 194, 197, 205, 208, 216, 219, 227, 230, 237, 240,248, 251, 259, 262, 270, 273, 281, 284, 291, 294, 302, 305, 313, 316,324, 327, 335, and 338. G.992.1 Annex C specifies similar boundarysymbol indices as does G.992.2 Annex C.

In one embodiment, the phase error determination module 604 uses alookup table stored in a conventional memory to identify which symbolsare boundary symbols. If the current symbol is a boundary symbol, thenaction 740 (FIG. 7) follows. Otherwise, the process 700 ends.

In action 740, the phase error is set to zero (0). The phase errordetermination module 604 ignores the phase error from the phase errormeasurer module 602 because the phase error was derived from a pilottone symbol that was sent during a noisy NEXT period or the pilot toneis otherwise unusable because it was sent as a boundary symbol in earlytraining mode. Boundary symbols are unusable in early training modebecause inter-symbol interference is present in early training, thus theFEXT boundary symbols are affected by neighboring noisy NEXT symbols.

Thus after process 700 concludes, the phase error determination module604 provides the phase error from the phase error measurer module 602 tothe PLL 516, or provides zero phase error to the PLL 516.Advantageously, the phase error determination module 604 ignores anyphase errors derived from low integrity symbols, i.e., symbols sentduring a NEXT period or of a boundary FEXT symbol in early trainingmode.

MODIFICATIONS

The foregoing description of the embodiments of the invention has beenpresented for the purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formdisclosed. Many modifications and variations are possible in light ofthe above teaching. It is intended that the scope of the invention belimited not by this detailed description, but rather by the claimsappended hereto.

1. An apparatus that determines a phase error to use to synchronize areceiver clock frequency with that of a transmitter, the apparatuscomprising: a transmitter that transmits at least one pilot tone symbolto the receiver; a sample clock that generates a sample clock signal; asymbol identifier coupled to receive the sample clock signal and thatindicates a symbol index associated with the most recent symbol, thesymbol index indicative of the crosstalk environment of the symbol; aphase error measurer device that measures the phase error between twopilot tone symbols and outputs the phase error; and a phase errordetermination device (“PEDD”) coupled to receive the phase error fromthe phase error measurer device and the symbol index from the symbolidentifier, wherein the PEDD determines whether to set the phase errorto zero based on the crosstalk environment of the most recent symbol,wherein the PEDD adjusts the sample clock signal frequency using thephase error.
 2. The apparatus of claim 1, wherein the PEDD determinesthe crosstalk environment of the most recent symbol by using the symbolindex and sets the phase error to zero if the crosstalk environment isnear end type.
 3. The apparatus of claim 1, wherein the receiver andtransmitter communicate in accordance with ADSL Annex C.
 4. A computersystem adapted for determining a phase error that is used to adjust aclock frequency, the system comprising: a module for recognizing atleast one pilot tone symbol; a module for indicating a symbol indexassociated with the most recent symbol, the symbol index indicative ofthe crosstalk environment of the symbol; a module for measuring phaseerror between two pilot tone symbols and for outputting the phase error;a module for determining whether to set the phase error to zero based onthe crosstalk environment of the most recent symbol; and a module forusing the phase error to adjust the clock frequency.
 5. The system ofclaim 4, wherein the module for determining determines the crosstalkenvironment of the most recent symbol by using the symbol index and setsthe phase error to zero if the crosstalk environment is near end type.